Abstract The effects of the substrate bias on the drain current and on the gate input capacitance of GaAs FET's are studied. The experimental results are explained : it is shown that a dual space charge and a positive fixed charge can develop at the N-epitaxial layer-semi-insulating substrate interface. Experimental methods for determining the technological and physical parameters of the N-layer, the interface and the substrate are proposed. It is deduced that the results obtained by using the classical C(V) profiling methods must be carefully analysed.